The present invention relates to a switching element substrate for liquid crystal display devices for transmissive display, a manufacturing method of the switching element substrate, and a liquid crystal display device.
Liquid crystal display devices (LCDs) have been actively studied and developed due to their advantages such as light weight, thin thickness, and low power consumption.
LCDs are divided into two types based on their operation principles: a simple matrix type and an active matrix type. Active matrix LCDs have active elements such as a plurality of thin-film transistors (TFTs) as switching elements for turning on or off each pixel. The active matrix LCDs are thus capable of sending a signal to each pixel independently. As a result, the active matrix LCDs have an excellent resolution and provide a clear image display.
The active matrix LCDs have a switching element substrate in which a plurality of switching elements are arranged in a matrix, a counter substrate facing the switching element substrate, and a liquid crystal layer provided between the switching element substrate and the opposing substrate. The active matrix LCDs display an image by driving the liquid crystal layer by the switching elements of the switching element substrate.
LCDs are also divided into two types: reflective LCDs for displaying an image by reflecting ambient light; and transmissive LCDs for displaying an image by transmitting light from a backlight (a light source). The transmissive LCDs are superior to the reflective LCDs in that the transmissive LCDs can display a high-quality image regardless of the brightness of the outside.
A conventional TFT substrate that is used in transmissive LCDs as a switching element substrate will be described with reference to FIG. 13. FIG. 13 is an enlarged cross-sectional view of a TFT formed on the TFT substrate.
The TFT substrate has a plurality of pixels arranged in a matrix, and a TFT 120 is provided for each pixel. Each pixel has a transmitting region that transmits light from the back side (i.e., from the bottom in FIG. 13).
Each TFT 120 is generally formed on a transparent glass substrate 101 capable of transmitting light (e.g., Japanese Laid-Open Patent Publication No. 7-240527). For example, a polysilicon film 106 is formed on the glass substrate 101. The polysilicon film 106 has a drain region 114, a source region 113, and a channel region 109 formed between the drain region 114 and the source region 113.
A gate oxide film 110, an insulating film, is formed on the polysilicon film 106. A gate electrode 111 is formed on the channel region 109 with the gate oxide film 110 interposed therebetween. The drain region 114 and the source region 113 are formed by implanting ions into the polysilicon film 106 at a high concentration by using the gate electrode 111 as a mask. It is also known in the art that an amorphous silicon film can be used instead of the polysilicon film 106.
A gate wiring (not shown) is connected to the gate electrode 111, and a source wiring (not shown) is connected to the source region 113. A pixel electrode (not shown) is connected to the drain region 114. The pixel electrode is provided in a transmitting region of each pixel, and is formed from a transparent electrode so that the pixel electrode can transmit light.
The liquid crystal layer is driven pixel by pixel according to a signal voltage applied to each pixel electrode through a corresponding source region 113 and a corresponding drain region 114. As a result, light from a backlight (not shown) located on the bottom side (i.e., on the back side) is selectively transmitted, whereby transmissive display is provided.
An SOI (Silicon On Insulator) substrate is known in the art. The SOI silicon is a silicon substrate having a monocrystalline silicon layer formed on a surface of an insulating layer. FIG. 14 is a cross-sectional view of a commonly used SOI substrate.
As shown in FIG. 14, an SOI substrate 130 generally has a sandwich structure. More specifically, the SOI substrate 130 has two monocrystalline silicon layers 131, 132 and a silicon oxide layer 133 (an insulating layer) formed between the monocrystalline silicon layers 131, 132.
There are two methods for producing an SOI substrate: an ion implantation method; and a substrate lamination method. In the ion implantation method, oxygen ions are implanted into a silicon wafer and the silicon wafer is then heat treated. For example, oxygen ions are implanted with an implantation energy of 180 keV and a dose of 1×1018 cm−2. In this way, an SOI substrate having a sandwich structure, an SOI substrate having a silicon oxide film formed at a prescribed depth of a silicon wafer, can be produced.
In the substrate lamination method, a silicon oxide film is formed on the surface of a silicon wafer, and another silicon wafer is laminated on the surface of the silicon oxide film. One of the silicon wafers is ground to a desired thickness. An SOI substrate having a sandwich structure can thus be produced.
When semiconductor devices such as TFTs are formed on an SOI substrate, TFTs can be formed from a monocrystalline silicon layer. Therefore, reduced parasitic capacitance and increased insulation resistance can be implemented as compared to the case where the above TFTs, TFTs formed from a polysilicon film or an amorphous silicon film, are used. As a result, the integration and the operating frequency of a device can be significantly increased.
However, a processing temperature of about 800° C. is required to form TFTs from a monocrystalline silicon layer although glass that is commonly used for a glass substrate has a relatively low melting point of about 600° C. Therefore, TFTs of a monocrystalline silicon layer cannot be formed on a transparent glass substrate.
TFTs of a monocrystalline silicon layer can be formed on an SOI substrate. However, an SOI substrate itself does not transmit light. Therefore, an SOI substrate cannot be used as a TFT substrate for transmissive LCDs.